Keto-enol tautomerism

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Keto-enol tautomerism

In organic chemistry

Organic chemistry

Organic chemistry is a discipline within chemistry which involves the science study of the structure, properties, composition, chemical reaction, and preparation of chemical compounds that contain carbon....
, keto-enol tautomerism refers to a chemical equilibrium

Chemical equilibrium

In a chemical process, chemical equilibrium is the state in which the Activity or concentrations of the reactants and products have no net change over time....
between a keto form (a ketone

Ketone

In organic chemistry, a ketone is a type of organic compound which contains a carbonyl group bonded to two other carbon atoms in the form:Neither of the substituents R1 and R2 may be equal to hydrogen ....
or an aldehyde

Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
) and an enol
Enol

Enols are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Enols and carbonyl compounds are in fact isomers; this is called keto-enol tautomerism:...
. The enol and keto forms are said to be tautomer

Tautomer

Tautomers are isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization. Commonly this reaction results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond....
s of each other. The interconversion of the two forms involves the movement of a proton

Proton

The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H+....
and the shifting of bonding electron

Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
s; hence, the isomerism qualifies as tautomerism.

A compound containing a carbonyl

Carbonyl

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double bond to an oxygen atom : C=O.The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex ; in this situation, carbon is triple-bonded to oxygen : C=O....
group (C=O) is normally in rapid equilibrium

Chemical equilibrium

In a chemical process, chemical equilibrium is the state in which the Activity or concentrations of the reactants and products have no net change over time....
with an enol tautomer, which contains a pair of doubly bonded carbon atoms adjacent to a hydroxyl

Hydroxyl

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Encyclopedia

In organic chemistry

Organic chemistry

Chemical equilibrium

In a chemical process, chemical equilibrium is the state in which the Activity or concentrations of the reactants and products have no net change over time....
between a keto form (a ketone

Ketone

Aldehyde

Tautomer

Proton

Electron

Carbonyl

Chemical equilibrium

Hydroxyl

Hydroxyl in chemistry stands for a molecule consisting of an oxygen atom and a hydrogen atom connected by a covalent bond. The neutral form is a hydroxyl Radical and the hydroxyl anion is called a hydroxide....
(−OH) group, C=C-OH. The keto form predominates at equilibrium for most ketones. Nonetheless, the enol form is important for some reactions. Furthermore, the deprotonated intermediate in the interconversion of the two forms, referred to as an enolate anion

Enolate anion

An enolate anion is an anion derived by loss of a proton from the alpha carbon of a carbonyl group; it is the anion of an enol.For instance, a base abstracts one proton from acetone to form enolate anion....
, is important in carbonyl chemistry, in large part because it is a strong nucleophile

Nucleophile

In chemistry, a nucleophile is a reagent that forms a chemical bond to its reaction partner by donating both bonding electrons. Because nucleophiles donate electrons, they are by definition Lewis bases ....
.

Mechanism

The conversion of an acid catalyzed enol to the keto form proceeds by a two step mechanism

Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs .Although only the net chemical change is directly observation for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism....
in an aqueous acid

Acid

An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
ic solution

Solution

In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent....
.

First, the exposed electrons of the C=C double bond of the enol are donated to a hydronium ion (H₃O⁺). This addition follows Markovnikov's rule

Markovnikov's rule

In organic chemistry, Markovnikov's rule or Markownikoff's rule is an observation based on Zaitsev's rule. It was formulated by the Russian chemist Vladimir Vasilevich Markovnikov in 1870 ....
, thus the proton is added to the carbon with more hydrogens. This is a concerted step with the oxygen in the hydroxyl group donating electrons to produce the eventual carbonyl group.

Second, the oxygen in a water molecule donates electrons to the hydrogen in the hydroxyl group, thus relieving the positive charge on the electronegative oxygen atom.

Erlenmeyer rule

One of the early investigators into keto-enol tautomerism was Richard August Carl Emil Erlenmeyer and his Erlenmeyer rule (developed in 1880) states that all alcohols in which the hydroxyl group is attached directly to a double-bonded carbon atom become aldehydes or ketones. This occurs because the keto form is generally more stable than its enol tautomer. As the lower energy form, the keto form is favored at equilibrium.

Significance in biochemistry

Keto-enol tautomerism is important in several areas of biochemistry

Biochemistry

Biochemistry is the study of the chemistry processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
. The high phosphate-transfer potential of phosphoenolpyruvate

Phosphoenolpyruvate

Phosphoenolpyruvic acid , or phosphoenolpyruvate as the anion, is an important chemical compound in biochemistry. It has the high-energy phosphate bond found in living organisms, and is involved in glycolysis and gluconeogenesis....
results from the fact that the phosphorylated compound is "trapped" in the less stable enol form, whereas after dephosphorylation it can assume the keto form. Rare enol tautomers of the bases guanine

Guanine

Guanine is one of the five main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, thymine, and uracil. In DNA, guanine is paired with cytosine....
and thymine

Thymine

Thymine is one of the four bases in the nucleic acid of DNA that make up the letters GCAT. The others are adenine, guanine, and cytosine. Thymine always pairs with adenine....
can lead to mutation because of their altered base-pairing properties .

In certain aromatic compounds such as phenol

Phenol

Phenol, also known as carbolic acid, is a toxic, white crystalline solid with a sweet tarry odor, commonly referred to as a "hospital smell"....
s the enol is important due to the aromatic character of the enol but not the keto form. Melting the naphthalene

Naphthalene

Naphthalene, also known as naphthalin, naphthaline, tar camphor, white tar, albocarbon, or antimite and not to be confused with naphtha, is a crystalline, Aromaticity, white, solid hydrocarbon with formula Carbon10hydrogen8 and the structure of two fused benzene rings....
derivative 1,4-dihydroxynaphthalene 1 at 200 °C results in a 2:1 mixture with the keto form 2. Heating the keto form in benzene

Benzene

Benzene, or benzol, is an organic compound chemical compound and a known carcinogen with the molecular formula Carbon6Hydrogen6....
at 120°C for three days also affords a mixture (1:1 with first order reaction kinetics) The keto product is kinetically stable and reverts back to the enol in presence of a base

Base (chemistry)

In chemistry, a base is most commonly thought of as an aqueous substance that can accept protons. A base is also often referred to as an alkali if OH- ions are involved....
. The keto form can be obtained in a pure form by stirring the keto form in trifluoroacetic acid

Trifluoroacetic acid

Trifluoroacetic acid is the most simple perfluorinated carboxylic acid chemical compound with the formula CF3CO2H. It is a strong carboxylic acid due to the influence of the three very electronegativity fluorine atoms....
and toluene

Toluene

Toluene, also known as methylbenzene or phenylmethane, is a clear, Water -insoluble liquid with the typical smell of paint thinners, redolent of the sweet smell of the related compound benzene....
(1:9 ratio) followed recrystallisation from isopropyl ether .

When the enol form is complexed

Metallocene

A metallocene is a compound with the general formula 2M consisting of two cyclopentadiene anions bound to a metal center in the oxidation state II....
with chromium tricarbonyl, complete conversion to the keto form accelerated and occurs even at room temperature in benzene.

DNA

In deoxyribonucleic acids(DNA), the nucleotide

Nucleotide

Nucleotides are molecules that comprise the structural units of RNA and DNA. Additionally, nucleotides play central roles in metabolism. In that capacity, they serve as sources of chemical energy , participate in cell signaling , and are incorporated into important cofactors of enzymatic reactions ....
bases are in keto form. However, James Watson

James Watson

James Watson is the name of:*James D. Watson , American biologist and co-discoverer of the structure of DNA*James Watson , British film and television actor...
and Francis Crick

Francis Crick

Francis Harry Compton Crick Order of Merit Royal Society , Ph.D., was a British molecular biology, physics, and neuroscience, and most noted for being one of the co-discoverers of the structure of the DNA molecule in 1953....
first believed them to be in the enol tautomeric form, delaying the solution of the structure

History of molecular biology

The history of molecular biology begins in the 1930s with the convergence of various, previously distinct biological disciplines: biochemistry, genetics, microbiology, and virology....
for several months .

Hydration of alkynes

Hydration

Hydration may refer to:* Hydration reaction, a chemical addition reaction* Mineral hydration, an inorganic chemical reaction where water is added to the crystal structure of a mineral...
of alkynes (of the general form RC=CR, where R is an alkyl

Alkyl

An alkyl is a univalent Radical consisting of carbon and hydrogen atoms, arranged in a chain. The Alkyls form homologous series with the general formula CnH2n+1....
group or hydrogen), produces an enol that is in equilibrium with the keto form. If R, R', or both are hydrogen atoms, the keto form is an aldehyde. If both R and R' are alkyl groups, the keto form is a ketone. The most commonly used set of reagents is sulfuric acid

Sulfuric acid

Sulfuric acid, hydrogen2sulfuroxygen4, is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry....
(H₂SO₄) and mercury(II) sulfate

Mercury(II) sulfate

Mercury sulfate, commonly called mercuric sulfate is the chemical compound Mercury SulfurOxygen4. It is an odorless solid that forms white granules or crystalline powder....
(HgSO₄).

Here, acetylene

Acetylene

Acetylene is the chemical compound with the symbol carbonhydrogen. It is the simplest alkyne.As an alkyne, acetylene is Saturation because its two carbon atoms are Chemical bond together in a triple bond....
(ethyne) is reacted with H₂SO₄ and HgSO₄, adding H to one carbon and OH to the other; this forms the intermediate enol. In this reaction, the carbons are equivalent and there is no stereoselectivity

Stereoselectivity

In chemistry, stereoselectivity is the property of a chemical reaction in which a single reactant forms an unequal mixture of Stereoisomerism during the stereospecificity creation of a new stereocenter or during the stereospecificity transformation of a pre-existing one....
. The reaction immediately continues with keto-enol tautomerization.

In general, the equilibrium lies far toward the keto side; in fact, the enol intermediate cannot be isolated as a product. Hydration of alkynes is unlike hydration of alkenes, where the product is an alcohol rather than an enol; therefore, no such equilibrium occurs.